Mark has it right, I think, it's an ST1, the A means it goes backwards (anti-clockwise). It's your basic common-or-garden air-cooled Lister, they probaby made about a billion of them, reliable as something that's very reliable, and plenty of parts available. Starting will be a standard starter motor with ring gear. You'll get 10 horse out of it, but at 2600rpm it'll be anything but quiet & refined...
$2200 is quite excessive for that engine IMHO. One sold here in the UK for less than £500 recently, and that came with a free dumper truck...